In the human eye, the precorneal tear film covering ocular surfaces is composed of three primary layers: the mucin layer, the aqueous layer, and the lipid layer. Each layer plays a role in the protection and lubrication of the eye and thus affects dryness of the eye or lack thereof. Dryness of the eye is a recognized ocular disease, which is generally referred to as “dry eye,” “dry eye syndrome” (DES), or “keratoconjunctivitis sicca” (KCS). Dry eye can cause symptoms, such as itchiness, burning, and irritation, which can result in discomfort. There is a correlation between the ocular tear film layer thicknesses and dry eye disease. The various different medical conditions and damage to the eye as well as the relationship of the aqueous and lipid layers to those conditions are reviewed in Surv Opthalmol 52:369-374, 2007 and additionally briefly discussed below.
As illustrated in FIG. 1, the precorneal tear film includes an innermost layer of the tear film in contact with a cornea 10 of an eye 11 known as the mucus layer 12. The mucus layer 12 is comprised of many mucins. The mucins serve to retain aqueous in the middle layer of the tear film known as the aqueous layer. Thus, the mucus layer 12 is important in that it assists in the retention of aqueous on the cornea 10 to provide a protective layer and lubrication, which prevents dryness of the eye 11.
A middle or aqueous layer 14 comprises the bulk of the tear film. The aqueous layer 14 is formed by secretion of aqueous by lacrimal glands 16 and accessory tear glands 17 surrounding the eye 11, as illustrated in FIG. 2A. FIG. 2B illustrates the eye 11 in FIG. 2A during a blink. The aqueous, secreted by the lacrimal glands 16 and accessory tear glands 17, is also commonly referred to as “tears.” One function of the aqueous layer 14 is to help flush out any dust, debris, or foreign objects that may get into the eye 11. Another important function of the aqueous layer 14 is to provide a protective layer and lubrication to the eye 11 to keep it moist and comfortable. Defects that cause a lack of sufficient aqueous in the aqueous layer 14, also known as “aqueous deficiency,” are a common cause of dry eye.
The outermost layer of the tear film, known as the “lipid layer” 18 and also illustrated in FIG. 1, also aids to prevent dryness of the eye. The lipid layer 18 is comprised of many lipids known as “meibum” or “sebum” that is produced by meibomian glands 20 in upper and lower eyelids 22, 24, as illustrated in FIG. 3. This outermost lipid layer is very thin, typically less than 250 nanometers (nm) in thickness. The lipid layer 18 provides a protective coating over the aqueous layer 14 to limit the rate at which the aqueous layer 14 evaporates. Blinking causes the upper eyelid 22 to mall up aqueous and lipids as a tear film, thus forming a protective coating over the eye 11. A higher rate of evaporation of the aqueous layer 14 can cause dryness of the eye. Thus, if the lipid layer 18 is not sufficient to limit the rate of evaporation of the aqueous layer 14, dryness of the eye may result.
For wearers of contact lenses, a widely reported ailment to physicians is intolerance to prolonged contact lens usage. Contact lens wear can contribute to dry eye. A contact lens can disrupt the natural tear film and can reduce corneal sensitivity over time, which can cause a reduction in tear production. In some patients, contact lens wear becomes unmanageable due to pain, irritation, or general decrease of visual acuity due to ocular discomfort. Typical remedies include repetitive eye drop applications, alterations of daily activity, or repeated removal of the contact lenses and return to standard eyeglasses or poor vision. For physicians, a typical treatment regime of revised medications and replacement contact lenses is tried and evaluated until a recommendation to alternative vision correction is employed for the patient. For many of these patients, evaporative dry eye disease is an underlying cause for their contact lens intolerance.
A system for determining which patients would be ideal candidates, or non-candidates for contact lenses, in the presence of ongoing dry eye disease would be of benefit to physicians and patients. Since dry eye disease can be an underlying problem as in the case of meibomian gland dysfunction (MGD) or non-obvious meibomian gland disease, a system that can be predictive of future contact lens problems would be advantageous. For MGD or meibomian gland disease in which dry eye signs are visible to the physician, a system that can qualitatively assess the potential impact of contact lens wear to the patient prospectively would be a benefit to future contact lens wear and the selection of model type.